In recent years, various methods have been tried to treat diseases of the internal organs, without applying major surgery, so as to lighten the physiological load on the patient and improve the progress in recuperation. Also, even if an abdominal operation is to be performed, the detailed determination of the state of diseased areas before surgery and determination of the internal state without having to cut open the organ surface during surgery would give very meaningful information at the time of actual surgery. To reply to these needs, in recent years, ultrasonography has developed and remarkably spread. The accuracy of the diagnosis using the ultrasonography before surgery has been extremely helpful in improving results of surgery in recent years. In particular, remarkable improvements have been made in the ability to diagnose diseases of the thyroid glands by the combination of the ultrasonography method and diagnosis of cells obtained by centesis and suction.
When trying to observe the internal state by placing the probe of the ultrasonograph directly on the body surface or organ surface, however, it is difficult to obtain a clear image in the region within several centimeters from the surface due to the nature of the ultrasonograph. Further, the actual body and organ surfaces are not flat, but have characteristic curves and unevenness, so it is impossible to bring an inflexible probe which maintains a certain shape into close contact with the desired location. That is, when air is present between the body and probe, the rate of propagation of the ultrasonic waves remarkably decreases, and an accurate image cannot be formed on the screen of the diagnostic apparatus.
To solve the above-mentioned problems, it is effective to interpose a suitable spacer (i.e., coupling medium) between the probe and body. The coupling medium is preferably made is the form of a sheet-like shape and is sandwiched between the probe and, for example, the body surface at the time of diagnosis or is formed into a suitable shape capable of being used by attaching to the probe directly or with a fitting. Such a coupling medium is required to have a suitable flexibility and mechanical strength and improved acoustic property (i.e., a low rate of attenuation etc.). For example, Japanese Unexamined Patent Publication (Kokai) No. 55-63636 discloses a specific water-containing polymer gel. The gel disclosed therein, however, has problems such as an insufficient mechanical strength or a large attenuation of sound waves. Various efforts have been made to deal with this problem. For example, there are known a polyvinyl alcohol based polymer gel (Japanese Unexamined Patent Publication (Kokai) No. 62-298342 and Japanese Examined Patent Publication (Kokoku) No. 2-46211), a highly water absorbing resin (Japanese Unexamined Patent Publication (Kokai) No. 4-53544), and various organic and inorganic polymers (Japanese Examined Patent Publication (Kokoku) No. 2-21252).
These various proposed polymer gels, however, also suffer from various problems. Namely, media using synthetic polymers suffer from the danger of all or part of the gel entering and remaining in the living body at the time of centesis or surgery. Thus, there are apprehensions over the toxicity of the gel itself or the residual monomers and therefore, there are problems in safety. Further, natural polymers and polyvinyl alcohol gels, which are considered highly safe, are not necessarily satisfactory in terms of their acoustic property (for example, the attenuation rate is high). To improve the acoustic property, it is necessary to increase the water content. However, when the water content is increased, there is the problem that the mechanical strength is decreased. Further, a polyvinyl alcohol gel tends to bleed water when pressure is applied and therefore is not suited as a gel for a probe used pressed against the human body or organ surface. Further, it is poor in sterilizability (that is, it completely melts and loses its original shape under heating at 121.degree. C. by an autoclave, which is one of the most common method for sterilization). As a result, it is not yet commercialized. In view of this situation, development of a coupling medium for a probe which is safe and can be used even during centesis and surgery has been desired.